mx03.masterlogin.de has no CNAME records.
Question and response
Question
dig mx03.masterlogin.de. NS
Answer
The response has no answer records.
Authority
masterlogin.de. 3428 SOA ns1.routing.net. hostmaster.masterlogin.de. 2026021100 86400 7200 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0